Having used Together & Rational I prefer Together but it is stupidly expensive (where as Rational is just ridiculously expensive).
However if you want 80% of the functionality for 5% of the price I would recommend Enterpise Architect
from Sparx Systems (see
http://www.sparxsystems.com.au/ea.htm).
You can download a fully functional evaluation version for free wiothout any of the sales hassle Together and Rational will put you through and the current price for a license is .
It exports and imports XMI model files so you always export your work into Ration or Together at a later date.
The developer is very approachable and involved with the product (as oppsed to trying to sell you consulting time like Together and Rational).
Cost is ~180 USD for 1 license, droping to around ~140 if you purchase in bulk.
